The drive towards higher density data storage on magnetic media has imposed a significant demand on the size and sensitivity of magnetic heads. This demand has been met, in part, by thin film inductive and magnetoresistive heads which can be fabricated in very small sizes by deposition and lithographic techniques similar to those used in the semiconductor industry. Thin film inductive heads are subject to the same problems as their core-and-winding predecessors of extreme sensitivity to gap irregularities and stray fields which result in output signal losses. Thin film magnetoresistive heads, on the other hand, rely on changes in the material's resistance in response to flux from the recording media, providing advantages of decreased sensitivity to speed of the recording media and higher density data capability. For these reasons, inter alia, magnetoresistive elements are increasingly preferred over inductive heads for reading data stored at high densities on magnetic media.
A figure of merit for magnetoresistive (MR) elements is .DELTA.R/R, which is the percent change in resistance of the element as the magnetization changes from parallel to perpendicular to the direction of the current. In the present technology, magnetoresistive elements are made from permalloy (81% Ni/19% Fe), which, at room temperature has a .DELTA.R/R of about 3%. For improved response and higher density data recording, a higher value of .DELTA.R/R is desirable.
In 1988 is was discovered that certain magnetic layered structures with anti-ferromagnetic couplings exhibit a phenomenon called "giant magnetoresistance" ("GMR") for which, in the presence of a magnetic field, .DELTA.R/R can be as high as 50%. The GMR phenomenon is derived from the reorientation of the magnetization in successive layers from antiparallel to parallel. This is distinctly different from anisotropic MR which depends on the relative directions of the magnetization and the measuring current. For optimum properties, the thickness of the multilayers must be less than 3 nm, and .DELTA.R/R increases with the number of pairs of thin film layers. Thus, these multilayers provide significant challenges for production because of the precision with which the thicknesses and other features, such as interface roughness, must be maintained for the many iterations of the pairs of magnetic and non-magnetic films. Several studies have shown that GMR oscillates in magnitude as a function of the thickness of the non-magnetic layers, increasing the concern about thickness control. These layered structures are also subject to output noise from magnetic domains, and, since their outputs are nonlinear, the devices must be biased to obtain a linear output. Most reported work has been on Fe/Cr superlattices, however, Co/Cr, Co/Cu and Co/Ru superlattices have also been found to exhibit GMR.
The extreme sensitivity to layer thickness places significant limitations on practical and economical application of GMR to data recording and other potential uses. Another significant obstacle to the practical application of GMR, such as in high density magnetic storage, is that the dramatic changes in resistance require relatively high magnetic fields to trigger the change, on the order of 250 Oersteds or more. These fields are too high for magnetic data storage, for which the saturation fields must be less than about 100 Oe.
An alternative to the multilayer structure of alternating magnetic and non-magnetic layers is the formation of magnetic particles in a non-magnetic matrix. One reported method for creating such a structure is to deposit alternating layers of magnetic and non-magnetic materials, then anneal the film to break up the layers into "islands" of magnetic material within a non-magnetic "sea". While this process may relieve some of the obstacles relating to extreme precision thickness requirements, the problem remains that very high magnetic fields are required to induce GMR.
The requirement of high magnetic field strength to achieve GMR results from, among other things, the magnetic anisotropy of the individual particles of magnetic material. If the particles have large shape anisotropy, or they are under high stress, they will be difficult to saturate. One approach to overcome this limitation is to use a magnetic material which forms spherical particles, i.e., decreased shape anisotropy. However, sufficiently low saturation fields to meet the needs of magnetic recording have not yet been attained.